The present invention relates to blood pressure measurement, in particular to a method and apparatus for the measurement of systolic blood pressure.
In most automatic indirect methods of blood pressure measurement, a pressure cuff is attached to a patient's arm adjacent a blood vessel, the cuff is pressurized with an applied pressure which is high enough to occlude the blood vessel and the applied pressure is gradually reduced. As the pressure is reduced to below systolic and then diastolic, blood begins to flow through the blood vessel creating the well known Korotkoff sounds and pulsatile pressures in the blood vessel. The sounds can be detected by a microphone at pulsatile pressures by a pressure transducer. The sensor, whether a microphone or pressure transducer, measures a quantity which is representative of the patient's blood pressure.
An oscillometric table is then formed of values of the quantity measured at various applied pressures as the applied pressure is gradually changed. Using the table the systolic and diastolic blood pressures are determined.
In a well behaved reading of blood pressure, the values generally increase from low values at applied pressures above the systolic to a maximum value at applied pressures between systolic and diastolic. Similarly, the values generally increase from low values at applied pressures less than diastolic to the maximum values. Respiration of the patient and other artifacts often cause the table to be non-monotonic. For example, respiration of the patient can change the instantaneous blood pressure by as much as forty millimeters of mercury. Effects of respiration on blood pressure are usually worse with people with compromised respiration and tends to be more pronounced at higher applied cuff pressures at or around systolic blood pressure levels.
Respiration then can affect the accuracy of blood pressure measurement particularly where a single threshold algorithm is applied to the oscillometric table in order to determine systolic blood pressure. Means for eliminating or minimizing the effects of respiration on the accuracy of measurement of systolic blood pressure is desireable.